间接碘量法测定铜合金中的铜（Determination of copper in copper alloy by indirect iodine method）.doc

间接碘量法测定铜合金中的铜（Determination of copper in copper alloy by indirect iodine method） Answers to experimental thinking questions How can 1. Na2S2O3 solution be formulated, can Na2S2O3 be dissolved in distilled water before boiling? Why? Answer: Na2S2O3 is not a standard substance, therefore, it is not possible to prepare standard solution directly. The prepared Na2S2O3 solution is unstable This is because the microorganisms in the water, CO2, react under the action of O2 in the air In addition, a small amount of Cu2+ or Fe3+ in water can also promote the decomposition of Na2S2O3 solutions 2. to potassium dichromate calibration Na2S2O3 concentration, why add KI, why in the dark for 5 minutes, before titration why dilute starch, why close to the end of the accession? Answer: (1) the role of KI has two points: first, the role of K2Cr2O7 and quantitative generation of I2, in order to demarcate Na2S2O3; The other is the synthesis of I3- complex ion with I2 complex to prevent the volatilization of I2 (2) when K2Cr2O7 and KI act, place 5 minutes in the dark for avoiding light, preventing I- from being oxidized by air, and facilitating the complete interaction of K2Cr2O7 and KI. Therefore, solutions should be applied Store in an iodine bottle or cone (covered with a surface dish), place in the dark for a certain period of time, and then titrate when the reaction is complete (3) to keep the acidity at calibration, dilute the solution at 0.2-0.4mol. L-1 before titration (4) because the I2 can partially oxidize the starch and make the end point advance and not obvious, the starch indicator should be added at the near end point when the indirect dosage method is adopted, that is, the I2 has been greatly reduced The solution is titrated and the solution is pale yellow when added to starch When measuring copper by 3. iodine method, why must pH remain between 3.5~4 and its influence is too low or too high? Answer: when measuring copper with iodine, the indirect iodine method is used. I